A Global Positioning System (GPS), which is a global navigation satellite system developed and promoted by U.S. Department of Defense (DOD), is essentially used in terrestrial, marine, and aerial fields for safe navigation. Such a GPS is also designated as a NAVSTAR/GPS from the standpoint of a system which uses a Navigation System with Time And Ranging (NAVSTAR) which is a navigation satellite system for medium/high orbits.
Such a GPS is composed of a total of 24 navigation satellites launched in groups of four satellites to six circular orbits having an altitude of 20,000 km, a period of about 12 hours, and an orbital inclination angle of 55 degrees, a ground control station for managing the satellites, and each user receiver.
Service interruption caused by the failure of a Global Navigation Satellite System (GNSS) may interfere with marine, aerial, and terrestrial traffic services, thus resulting in problems such as economic loss. Therefore, during the provision of a GNSS service, there is a need to execute Integrity Monitoring (IM) for a user.
Network Real Time Kinematic (RTK), which is augmentation technology based on the carriers of multiple reference stations, has been mainly utilized in geodetic survey fields requiring centimeter-level positioning performance in an early stage, but has recently evolved into technology aiming at improving positioning performance in dynamic systems, such as Intelligent Transportation Systems (ITS), precision approach, and harbor navigation systems.
Technology aiming at improving accuracy which is considered to be important in geodetic survey fields has been continuously executed, but technology related to integrity, continuity, and availability, which are other performance request elements required to be utilized in application fields for positioning, is still insufficient.
In dynamic systems aiming at positioning, technology for integrity, continuity, and availability, in addition to accuracy, is required so as to actually implement and apply network RTK.
The causes of anomalies in a GPS satellite may include anomalies in satellite clocks, anomalies in satellite orbits, anomalies in navigation messages, etc., and such anomalies in the GPS satellite increase an error in the pseudo-range of the corresponding satellite, exhibit an instable state in code and carrier phase measurements, and cause discontinuous points, thus resulting in which a positioning error increases.
From the standpoint of a user, there is a problem in that a known anomaly in a GPS satellite may be replaced, but the stability of a positioning service is deteriorated in the case of an unknown anomaly, thus making it impossible to provide the service.
Conventional technologies capable of determining whether an anomaly occurs on a satellite include a Receiver Autonomous Integrity Monitoring (RAIM) technique used by a single user receiver, Reference Station and Integrity Monitors (RSIM) operated by a Differential GPS (DGPS), a 3-QM (Quality Monitoring) technique operated by a Local Area Augmentation System (LAAS), etc.
The RAIM technique independently used by a single user receiver itself includes a method of comparing pseudo-ranges and position solutions, a method of comparing the maximum separation distances of navigation solutions, a method using parity vectors, etc. Most of these methods have the concept of determining the presence or non-presence of anomalies using redundant measurements, and are problematic in that there are many restrictions in use and reliability is low.
Further, as disclosed in “development report for a dual frequency receiver for DGPS reference stations (in 2006, Ministry of Maritime Affairs and Fisheries, Sang-Hyeon Seo et al.),” RSIM operated by DGPS is intended to guarantee the integrity of generated code-based correction information, and is capable of analyzing a difference between a measured pseudo-range and a predicted pseudo-range using characteristics that the location of the receiver of a DGPS reference station is exactly known, and is then determining whether an anomaly is present.
Such a determination method can individually detect the presence or non-presence of anomalies in all observed satellites, but is problematic in that code measurements are used, and required performance is low, and thus it is not suitable for application to network RTK.
Further, 3-QM operated by LAAS is chiefly divided into signal QM (SQM) for examining signal anomalies caused by a GPS satellite and a sudden variation in the ionosphere occurring during a radio wave propagation, data QM (DQM) for examining an error in navigation messages, and Measurement QM (MQM) for examining the measurements of the receiver, wherein SQM mainly aims at detecting an Evil Waveform (EWF), signal power errors, and ionospheric divergence. In order to apply such SQM, a procedure for re-designing a receiver in conformity with the necessity thereof is required.
DQM mainly aims at detecting code errors, bit errors in navigation signals, and errors in orbital information, and MQM mainly aims at detecting errors in code and carrier phase measurements, and performs filter divergence examination, measurement variation examination, sigma value examination, and B-value examination. 3-QM is a method of normalizing specifications and performance so that it is suitable for an LAAS system.
Therefore, even in a network RTK environment, the development and application of a suitable anomaly monitoring processing technique are currently required.